The invention relates to a control cone for control valves, in particular angle control valves, for critical operating states resulting from cavitation or flashing of abruptly vaporising and erosive media, the control valve comprising a valve seat and a valve stem with a conical valve head guided longitudinally movably therein.
In many processes control valves are used to control level or pressure in an upstream reactor/vessel. The pressure in the valve inlet Pi is roughly the same as the pressure in the upstream reactor/vessel, since this is in balance with the gas mixture above the liquid in the upstream reactor/vessel. Pi is accordingly roughly the same as Pv, the vaporisation pressure of the medium. Thus a situation arises in the control valve on the lower-pressure side in which the outlet pressure Po is a long way below Pv.
It is thus certain that the medium will vaporise or degas as soon as the pressure drops in the control valve. This is the case once the medium has passed through the valve seat. As the medium vaporises, it needs significantly more space. The medium would require approximately 1000 times more space, since the mass flow rate remains constant and the density of the gas is approximately 1000 times lower than the density of the liquid. Since there are no good possibilities for expansion in a normal straight-way valve, the control valve is obstructed and cavitation in the medium helps to destroy the materials of the valve seat, valve cone and valve housing.
A first remedy involves the use of an angle control valve, in which the medium can expand and accelerate unimpeded downstream. However, this also means that the medium “shoots” at very high speed through the downstream pipe and may possibly destroy everything lying in its path. This could also lead to destruction of the downstream reactor/vessel.
A further possible remedy consists in a per se known angle control valve according to FIG. 1 with a control cone which opens outwards, i.e. out of the valve. This only functions if the control valve is mounted directly on the inlet flange of the downstream reactor/vessel. The control cone then opens directly into the reactor/vessel, such that the liquid may expand through vaporisation, since, compared with the pipe, the vessel offers vastly more space for expansion. This significantly reduces acceleration of the medium downstream of the control valve.
However, there is generally high lateral pressure on the control cone, so meaning that the latter has to be guided. This is achieved by a plurality of ribs (FIG. 2), which extend coaxially outwards from the front edge, i.e. from the wider end of the conical face of the valve head, over the tapered end thereof as far as into the front portion of the valve stem. However, in practice it is noted that these ribs disturb flow around the conical valve head, such that erosion occurs to a greater extent around the ribs in the valve seat and at the conical face.
The object of the invention is to provide a control cone for control valves of the above-mentioned type by means of which flow of the medium can be better distributed and localised erosion in the valve seat can be eliminated.